Commentary

Space Travel in Sci-Fi

The original Star Wars knocked me back to reality when Hans Solo commented that his spacecraft traveled a certain interstellar route in several parsecs. Parsecs?! But the parsec is a unit of distance, not time. It took several minutes before I got back into the movie.

I enjoy science fiction movies, but not for their science. I enjoy them for what they say about the contemporary society of the filmmaker. I can as easily watch Things to Come, with its 1930s vision of unconditional air warfare and communist utopia as I can Blade Runner, with its 1980s vision of killer robots in a dystopian Los Angeles of environmental degradation and overpopulation. The first movie shows the attraction of that era's society to communism and fascism. The second film, along with other contemporary films, many of which appear to be shot in the Los Angeles dump, show a deep unease about the future, fostered in part by the inflation and unemployment of the late 1970s and early 1980s, by pessimism over the outcome of the Cold War, and perhaps by Hollywood's dislike of President Ronald Reagan.

But what to say about the science of these and other films? Things to Come, which is based on a book by H.G. Wells, comes remarkably close to getting the technology right. As for science, the film contains none. The technology of Blade Runner is fanciful, and most likely it is off the mark, because human behavior is a consequence of evolutionary change, with one layer of behavior superseding earlier layers at each evolutionary stage. A thinking robot would have a different evolutionary past. The science is also fanciful, both for the movie's floating cars and for the suggestion that interstellar travel is an easy undertaking.

Easy interstellar travel is a feature of most science fiction movies and books, not to mention of the UFO mythologies. From the standpoint of modern physics, however, easy interstellar travel is impossible. The distances between the stars are immense, and the speed at which one can travel is limited to less than the speed of light.

The closest known star to the solar system is Proxima, also known as V645 Centauri, at 1.3 parsecs (4.2 light years, or 410¹⁸ cm). There are only 11 known stars closer than 10 parsecs to the solar system, and six of these form three binary systems. Even at the speed of light, travel to one of these 11 stars would take from 4 to 30 years from the standpoint of those waiting on Earth. With that travel time, its a good thing that Yoda has such a long life span; otherwise, he would have died of old age waiting for Luke to arrive.

To get out of this problem, many science fiction writers invent new physics. The writers for the excellent (for the first-four years at least) television series Babylon Five solved the problem by having a spacecraft enter a parallel universe with shorter dimensions (or perhaps higher speeds) called ?hyperspace?. Frank Herbert in his drug-mysticism pervaded Dune created a world in which a spacecraft pilot gives himself the ability to ?fold? space, which allows him to transport his spacecraft from one point to another instantaneously, by ingesting a ?spice?. Other writers invoke faster-than-light travel or shortcuts in space such as worm-holes to get around the speed-of-light limit.

But these solutions actually only address one feature of special relativity; special relativity has a second feature that is only well known to students of physics: events that appear simultaneous to one person may not necessarily appear simultaneously to another person. What is meant by this? If I see two lights separated in space turn on, and if I know the distance to the lights, I can calculate the time when they turned on by taking into account the light-travel time. Let us say I find that they turned on simultaneously. Someone moving parallel to the line defined by the two lamps would find from a similar calculation that the lamp ahead of him turns on before the lamp behind him. This surprising behavior is a necessary feature of special relativity, and this feature eliminates faster-than-light travel from our physics. Why? Taking Herberts drug-induced teleportation as a concrete example, if I can move to a new point in space that is simultaneous in time with myself, then I can adjust the time at the new point either forward or backward by an arbitrary amount by simply changing my speed before teleporting. In effect, I can time travel by teleporting to a place, changing my speed at that place, and then teleporting back to where I started. From this we see that we cannot beat the speed-of-light limit without introducing paradoxes into physics.

There is no hope for Yoda: he must wait decades for Luke to reach him. But Luke does have a real solution for himself. In special relativity, time passes more slowly for someone who accelerates. For the mathematically inclined, the time t measured by someone at rest is related to the time measured by an accelerating traveler τ by the equation t = c sinh( aτ/c )/a = 0.97 sinh( 1.03τ_yr ) yr, where c is the speed of light and a is the acceleration. In the last form of the equation, the time is given in years and the equation is specialized to an acceleration equal to the gravitational acceleration at Earth's surface (a=g). After a year of traveling at 1g, time for our traveler passes significantly more slowly than for those at rest. For instance, 1 year of time for our traveler equals 1.2 years for those at rest, 2 years for the traveler equals 3.8 years for those at rest, and 5 years for the traveler equals 84 years for those at rest. A round trip to the center of our Galaxy from Earth, a total distance of 15000pc, takes about 49,000 years as measured for those on Earth, but for the traveler accelerating at 1g, this trip takes only 11 years. So Luke can reach Yoda while still a young man.

The only movie I'm aware of that takes advantage of this aspect of special relativity is the original Planet of the Apes; the movie begins with the astronaut Taylor (Charlton Heston) explaining this time dilation, and the ending of the movie turns on this effect. But the movie attributes the theory to a fictitious physicist rather than to Albert Einstein! Even when it's right, it's not quite right.